1. Field of the Invention
The present invention relates to a gas discharge display, and more particularly, to a gas discharge display for use in a PDP(Plasma Display Panel), in which a discharge region is filled with 4 kind of gases mixed at a particular ratio, for improving a reliability of a product.
2. Discussion of the Related Art
In general, there are a DC type, an AC type, and a hybrid type, a combination of the DC type and the AC type, in the gas discharge display in view of electrode structure. The DC type and the AC type differ in that whether the electrode is exposed directly to discharge plasma or indirectly through a dielectric layer. In the case of the DC type PDP, the electrode is exposed directly to the discharge plasma, and, in the case of the AC type PDP, the electrode joins with the discharge plasma indirectly through the dielectric layer. This difference leads to show a difference in discharge. In the case of the AC type PDP, charged particles formed from discharges are accumulated on the dielectric layer. That is, electrons are accumulated on the dielectric layer over an electrode having a positive potential charged thereto, and ions are accumulated on the dielectric layer over an electrode having a negative potential charged thereto. A potential formed by this phenomenon is called as a wall potential, which has a polarity opposite to an external potential, to lower a potential applied to a gas in a cell once the wall potential is started to form. Accordingly, an adequate wall potential is formed, the potential applied to the gas is lowered below a level at which a sustaining of the discharge is possible no more, the discharge is canceled. However, if the polarity of the potential applied to the external electrode is changed after the wall potential is formed, the potential applied externally will be added on top of the wall potential, allowing the AC type PDP operative according to a memory function in which a discharge can be made even if a low external potential is applied. Thus, the AC type PDP has the memory function due to the wall potential accumulated on the dielectric layer. That is, a cell having a discharge made previously to form a wall potential on the dielectric in the cell can make a discharge at a voltage lower than a cell without the wall potential. This memory function is very useful characteristic for operating a large sized PDP, a gas discharge display employing line driving system. Different from the AC type PDP, since the DC type PDP has no function of the wall potential formation on the dielectric, it has no intrinsic memory function. That is, as the electrode is directly exposed to a discharge region, charged particles from the discharge flow to external circuits through electrodes of opposite polarities, without any accumulation of the charged particles on electrode surfaces. However, in the case of the DC type PDP, a pulse memory function in which a charged particle supply effect is employed is used. The pulse memory function employs a principle that a discharge can be made at a voltage lower than a case when there are no charged particles, and quasi neutral particles if a discharge pulse is applied before the charged, and quasi neutral particles formed from a previous discharge are attenuated. This memory function is an essential feature for allowing operation of a large sized panel in the line driving system without loss of luminance and is also required in view of an electrode structure.
FIGS. 1 and 2 illustrate sections showing basic electrode structures of the DC type PDP and the AC type PDP, respectively.
Referring to FIG. 1, the basic electrode structure of the DC type PDP is provided with an anode 3 and cathode 4 on a front substrate 1 and a back substrate 2 respectively, barrier ribs 5, and a fluorescent material layer 6. The anode 3 and the cathode 4 forms a current path for forming a discharge. The barrier rib 5 fixes a distance between electrodes for forming the discharge, and prevent a crosstalk caused by a discharge in an adjacent cell. In the DC type PDP, nickel is mostly used as an electrode material, which has a high secondary electron emission coefficient for providing a low discharge voltage characteristic and an excellent anti-sputter characteristic endurable on ion sputtering. And, the AC type PDP shown in FIG. 2 is provided with a dielectric layer 10 each covering sustain electrodes 7 and 8 and an address electrode 9 for forming a capacitance coupled discharge. In general, the dielectric layer is formed of a material to selected from borosilicate group coated with a thin film of oxide, such as magnesium oxide, as a protection film 11 because the dielectric layer 10 of borosilicate group has a lower secondary electron emission coefficient and a short lifetime against sputtering of ions in the plasma. The magnesium oxide MgO has a good anti-sputter characteristic, but also a high secondary electron emission coefficient that provides a low voltage characteristic. However, since the magnesium oxide layer should be thin and have an excellent surface characteristic, the magnesium oxide layer is in general formed by a thin film formation process of vacuum deposition, rather than thick film formation in printing. The barrier rib 5 is required to have a height of 100.about.200 .mu.m for maintaining discharge distance and volume. As one layer of thick film printing is a few tens of .mu.m, the barrier rib 5 may be form by multi-layers of the thick film printing. And, though a number of electrodes required for a discharge is two, in general an electrode structure with three electrodes is mostly used. The DC type PDP has an additional supplementary cathode for forming a supplementary discharge, and the AC type PDP is introduced of an address electrode 9 for separating the sustain electrodes 7 and 8 from a selective discharge and sustain discharge to improve an address speed. Accordingly, the electrode structures may be classified as a two electrode structure and a three electrode structure according to a number of electrodes. Or, the electrode structures may be classified as an opposite type electrode structure and a surface discharge type electrode structures. In the opposite type electrode structure, two sustain electrodes for occurring a discharge are disposed on the front substrate and the back substrate respectively to cause a discharge formed vertical to the panel, and, in the surface discharge type electrode structure, the two sustain electrodes for occurring a discharge are disposed on the same substrate, to form a discharge on one plane of the panel.
FIG. 3 illustrates a perspective view of a background art AC type PDP, provided with a front substrate 1 form of glass for easy transmission of a light, sustain electrodes 7 and 8 each composed of a transparent electrode and a metal electrode on a top surface of the front substrate 1 disposed in a transverse direction at fixed intervals for sustaining a discharge voltage, a dielectric layer 10 formed on an entire surface of the front substrate 1 inclusive of the sustain electrodes 7 and 8 for protecting the sustain electrodes 7 and 8, a protection layer for protecting the dielectric layer 10 to prolong a lifetime of the dielectric layer 10, improving a secondary electron emission effect, and reducing variation of a discharge characteristic, a back substrate 2, a lower base film 12 on an entire top surface of the back substrate 2, address electrodes 9 on the lower base film 12 formed in a direction perpendicular to the sustain electrodes 7 and 8 at fixed intervals, a white back 13 formed on an entire surface of the lower base film 12 inclusive of the address electrodes 9, barrier ribs formed between, and in parallel to underlying address electrodes 9 for maintaining a space between the front substrate 1 and the back substrate 2 and preventing unwanted discharge between cells, and R, G, B fluorescent material layers 6 formed between the barrier ribs 6. Upon finish of fabrication of the front substrate 1 and the back substrate 2, an air drawing hole is formed in the back substrate 2 for filling a desired discharge gas at a vacuum once fabrication of the front substrate 1 and the back substrate 2 are completed. Then, bonding frit is applied at rims of the front substrate 1 and the back substrate 2, and the front substrate 1 and the back substrate 2 are bonded together, to form a discharge region between the front substrate 1 and the back substrate 2. A discharge gas of a desired light characteristic is filled through the air drawing hole, and the hole is sealed, to complete a PDP fabrication.
In the meantime, in order to display colors in the PDP, a principle the same with the CRT is employed, in which fluorescent materials are excited. Though an electro-luminescence of an electric field excited by electrons accelerated to a few ten KeV is employed in the case of a CRT, a photo-luminescence, an excitation of fluorescent material by an ultra-violet ray from a gas discharge, is employed in the case of the PDP. Particularly, a 147 nm vacuum UV ray of Xenon gas is mostly used. Accordingly, fluorescent materials are coated on the PDP electrode structure for displaying colors.
Also, there are a transmissive type of PDP electrode structure and a reflective type of PDP electrode structure depending on locations of coating of the fluorescent materials. Though the transmissive type of PDP electrode structure is simple in fabrication, there are a great variation depending on a printed surface condition of the fluorescent material, and, though the reflective type PDP electrode structure can enlarge an area of fluorescent material coating with an increased luminance, coating of the fluorescent material is difficult. The reflective type PDP electrode structure has a higher luminance than the transmissive type PDP electrode structure, and as the difficulty in coating the fluorescent material has also been solved according to the development of thick film printing technology and new development of technology such as sandblasting, the reflective type PDP electrode structure is widely used, currently.
As explained, the gas discharge display applied mostly to the PDP, being a core technology for displaying an image, utilizes the Penning Effect. The Penning Effect is a reaction in which an ionization is enhanced through species having very large collision section, wherein an .alpha.-process of Thounsand is increased.
For example, the Penning Effect of He+Xe and Ne+Xe is as follows. EQU He*+Xe.fwdarw.Xe++e+He EQU Ne*+Xe.fwdarw.Xe++e+Ne
Where He and Ne are major gases, Xe is an additive gas, and He* and Ne* are quasi stable or excited particles of pertinent particles. Because each of these excited particles has relatively long life time with a large collision sectional area, with a greater probability of collision to other particles as much, the ionization is enhanced from the influence of these neutral excitestables when He, and Ne gases are added compared to the case when there is Xe gas only in the aforementioned equation.
Together with this, the secondary electron emission from a surface of material the plasma is in contact with is very important to the discharge characteristic of the PDP. Particularly, the secondary electron emission caused by plasma from the electrode covered with dielectric as in the case of the AC type PDP may come from direct ion impact, surface reaction of metastables, and reaction caused by light and the like, of which major one is the reaction from ion impact. A neon ion with an ionization energy 21.6 eV is incident, to couple with one electron in a valence band and be neutralized, to cause a surplus energy to discharge an electron in another valence band to a surface. In this instance, a motion energy of the electron can be obtained by subtracting a band gap energy and a surface work-function energy of MgO from an incident ion energy. The electron is accelerated in field, and generates an ion and electrons in a plasma state when a collision happens.
Next, VUV in the gas discharge display will be explained.
The VUV is UV of a wavelength below 200 mm. The VUV can not pass through a gas, but is heavily absorbed to the gas if a pressure of mother gas is high, or oxygen is contained therein. A wavelength and an intensity of VUV are important factors that determines a luminance of a light emitted from the PDP. An UV ray from Xenon Xe has a wavelength ranging 140 mm.about.180 mm, which is consistent with a wavelength region in which R, G, B fluorescent materials give the best efficiency. Of inert gases, since helium He and neon Ne emit light of a short wavelength below 100 nm, helium He and neon Ne are not suitable for use as gases which emit an UV ray that excites the fluorescent materials to emit visible light. Taking an intensity and a wavelength of an emit UV ray into account, though xenon Xe appears suitable for use as a PDP gas, a mixture of two or three gases are generally used because a driving voltage or a lifetime of the electrode should be considered for using a gas in the PDP on the same time, typical one of which mixture of two gases is helium He or neon Ne added with xenon Xe, which lowers a driving voltage and improves an UV efficiency. As such, helium He or neon Ne is used as a major gas, because an excitation of xenon is efficient due to a higher temperature of electron in the gas compared to a pure xenon and a Penning effect of xenon can be used. And, even in the case of mixture of gases, conditions giving the maximum UV efficiency may be different depending on a ratio of mix and other discharge conditions.
FIG. 4 illustrates a graph showing UV ray intensity from a DC type cell vs. pressure of He+Ne gas therein, wherefrom it can be known that, as the pressure becomes higher, a DC type cell of which positive column is adapted to be used as a major luminous region is involved in drop of the UV intensity, and DC type cell of which negative glow is used as a major luminous region is involved in rise of the UV intensity, i.e., it can be known that a trend according to a partial pressure of xenon Xe may differ depending on a structure of a discharge cell.
FIG. 5 illustrates a relative wavelength of xenon vs. a pressure of He+Xe(7%), wherefrom it can be known that as a pressure of He+Xe(7%) gas become higher, an intensity of light with a wavelength of 173 nm emitted from Xe.sub.2 * is increased while an intensity of UV ray with a wavelength of 147 nm emitted from Xe.sub.2 * is decreased. This is because dimers, molecular particles, can be produced with ease as the pressure become higher. In view of a distance from an electrode, a luminance in a negative glow region near to a cathode surface is the highest, and much light is emitted from a positive column region as a distance from the electrode becomes far. The negative glow and the positive column are compared that, though the luminance in the negative glow is higher, a portion of the positive column is very large if a total amount of light emitted per a unit time period is considered. Especially, while the negative glow region is limited, since the positive column region is increased as the electrodes distanced far, the positive column region may give a dominating influence depending on an electrode structure. Xe*(.sup.3 P.sub.1) is produced in plasma through a path including excitation by an electron, formation of xenon molecule ions by recoupling, transition to a resonance state of particles in a quasi stable state by collision, and so on. And, Xe.sub.2 *(173) is produced by three of xenon collision of Xe* to neutral particle. Thus, as a composition and a pressure of gas can make a wavelength and an efficiency of light different, and a cell structure or a driving circuit indirectly affects to this, determination of an optimal discharge gas should be made in combination with a cell structure and a driving circuit.
FIG. 6a illustrates data on a spectrum of light emitted from neon gas singly sealed in a gas discharge display, FIG. 6b illustrates data on a spectrum of light emitted from xenon gas singly sealed in a gas discharge display, and FIG. 6c illustrates data on a spectrum of light emitted from argon gas singly sealed in a gas discharge display.
Referring to FIG. 6a, all the light emission of neon in a range of 5800.about.7000.ANG. are caused by transition from 3 p(18-19 eV) to 3 s, as shown in FIG. 6b, xenon shows an intense light emission both in infrared ray region and ultra-violet region, while a weak light emission in a visible light region. Also, an ion line can be observed at an excitation level of approx. 11 eV because xenon has a low ionization voltage of 2.12 eV and a high discharge voltage (approx. 250V). And, as shown in FIG. 6c, the light emission of argon in the vicinity of 7000 .ANG. is caused by transition from 5 p(approx. 14 eV) to 3 s. FIG. 7 illustrates a spectrum of a mixture of two gases based on neon added with xenon(1%). The Ne+Xe, a Penning gas, shows a discharge voltage lower than neon only. That is, once argon gas is added to an existing discharge cell, ionization and excitation are done efficiently, not only to form a high charged particle density, but also to increase excitation in which a vacuum UV ray is emitted. As a result, emitted vacuum UV ray is increased, that induces excitation of the fluorescent materials with an improvement of luminance and a reduction of a driving voltage for causing the discharge, which improves an efficiency.
FIGS. 8 and 9 illustrate test results on mixture of three gases, addition of argon gas to an existing mixture of two gases. If FIGS. 8 and 9 are reviewed in combination, it can be known that a discharge starting voltage is lowered and a vacuum UV ray emission is increased when argon gas is added thereto at a ratio of approx. 0.001%.about.1.0% compared to the case when a mixture of two gases is used. FIG. 8 illustrates measurements of discharge starting voltage vs. a composition ratio of argon gas in a mixture of Ne+Xe+Ar, wherein it can be known that the discharge starting voltage is reduced when argon is mixed up to a composition ratio 0.001%.about.1.0%, particularly at 0.3%.about.0.7%, with the best efficiency at 0.5% in the mixture of Ne+Xe+Ar. FIG. 9 illustrates an amount of vacuum UV ray vs. a composition ratio of argon gas in the mixture of three gases of Ne+Xe+Ar. As shown, an amount of vacuum UV ray at a composition ratio of 0.001.about.1.0% of argon in the mixture of Ne+Xe+Ar is greater than the mixture of two gases Ne+Xe, and as shown in FIG. 8, the vacuum UV ray is much more in a composition ratio of 0.3.about.0.7% of argon, and the most at a composition ratio of 0.5. FIG. 10 illustrates discharge starting voltage and sustain voltage vs. a composition ratio of xenon in a mixture of He.sub.x +Ne.sub.1-x : x=0.7 at a fixed pressure of 500 torr. Though a composition ratio used is within a range of 0.001.about.10%, as shown in FIG. 10, the voltage reduction is shown at a composition ratio of 0.001.about.4%, with a peak of the voltage reduction at 0.001.about.2% and the best at 1%.
However, a mixture of two or three gases in place of a single gas used for dropping the discharge starting voltage has caused a luminance drop, and basically, a display with the mixture of two gases has a luminance lower than display of CRT. And, though xenon gas generates the longest wavelength of inert gases in terms of UV ray, xenon gas can not be used singly due to too high discharge starting voltage, but used together with a mixture of two gases of neon or helium to drop the discharge starting voltage, which however causes a problem of degradation of color purity and lifetime. That is, as shown in FIG. 11, because neon+4% xenon exhibits a degradation of color purity as a color coordinate `X`-axis shows "0.6" due to orange visible light, and helium+4% xenon has a problem of short lifetime, [He:Ne(7:3)]+4% xenon, or [He: Ne(8:2)]+4% xenon is used for improving the color purity. However, as shown in FIG. 12, as [He:Ne(8:2)]+4% xenon shows a discharge luminance of 6 cd/m.sup.2 and the [He:Ne(7:3)]+4% xenon shows a discharge luminance of 12 cd/m.sup.2, an effect of a color purity improvement higher than a certain level can not be expected. Thus, though a mixture of three gases of helium, neon and xenon has been used for improving degradation of color purity when a mixture of two gases is used in a background art PDP, the mixture of three gases could not satisfy essential conditions of a plasma display panel of a low discharge starting voltage, a high luminance, improved color purity and a long life on the same time.